Men who have sex with men (MSM) experience rates of HIV and bacterial sexually transmitted infections (STI) that are many times those experienced by heterosexuals in the U.S. Despite the widely acknowledged importance of sexual behavior in defining HIV/STI transmission dynamics, little is known about how MSM form patterns of sexual behavior, what factors predict these patterns, and how those behavioral patterns change over a person's life. We propose to conduct a cohort study of 100 MSM who are: 1) age 16-20 with 1-3 lifetime male sex partners, or 2) age 16-30 within 3 years of their same-sex sexual debut. This cohort will be followed for 1 year during which participants will complete a baseline computer administered self-interview (CASI) regarding early homosexual experiences and sexual behavior;retrospective online follow-up questionnaires every 3 months;web-based sex diaries;and HIV/STI testing at baseline, 6, and 12 months. The study's first aim is to define the feasibility of enrolling a diverse sample of MSM near the time of same-sex sexual debut, following the cohort for 1 year, collecting frequent sexual behavioral data, and serially testing these men for HIV/STI. Success will be defined based on the number of men who enroll and complete the study, and the proportion who complete all questionnaires and HIV/STI testing. The study's second aim is to assess the feasibility of using web-based sex diaries to collect sexual behavior data and to identify the appropriate diary schedule. MSM will be randomly assigned diary schedules and we will compare the frequency of reported behaviors across diary schedules and against the retrospective questionnaire data. Finally, for the third aim we will conduct three analyses. First, we will asses the association between early sexual development and the context of initiation of contact with the gay community. Second, we will investigate the association between the context of same-sex sexual debut and risk behavior observed throughout the one year of follow-up. Third, investigators will conduct exploratory analyses to define patterns of sexual behaviors such as condom use, HIV status disclosure, and sexual role and repertoire. These analyses will assess how risk varies between individuals and within individuals (e.g., by partnership type), and will seek to establish typologies of risk that can be longitudinally measured in a future, larger and longer cohort study. Findings from the proposed study, as well as those from a larger and more definitive cohort study of MSM sexual behavior, could help define HIV/STI transmission dynamics;identify factors in early homosexual experiences that predict subsequent risk;provide insights into the development of interventions to prevent HIV in MSM;and provide a platform for future prospective intervention studies. PUBLIC HEALTH RELEVANCE: In 2005, MSM (including MSM/IDU) comprised 53% of new HIV/AIDS cases in the U.S (CDC, 2005). Current scientific and prevention efforts among MSM are hampered by the paucity of knowledge about how early homosexual experiences affect subsequent risk, and how the sexual behavior of MSM changes over a person's lifetime. We propose a pilot cohort study of young, sexually inexperienced MSM to define the feasibility of conducting a large, prospective cohort study of young MSM that might define the natural history of male homosexuality. In addition to informing the design of this larger study, findings from the proposed research could provide empirically derived parameters for mathematical models, assist in the design of prevention programs for MSM, and identify the highest risk young MSM for future interventions.